This invention relates to a hydrocarbon leak detection system used to test for leaks in a liquid product line. More specifically, this invention controls a pump to pressurize a liquid product line to compensate for thermally induced pressure changes during a product line leak test.
A typical use for a hydrocarbon product dispensing line pressure probe is in a vehicular service station that dispenses fuel. In a vehicular service station, it is common for fuel to be stored in large underground storage tanks and pumped through a product dispensing line to a dispenser. The dispenser typically has a switch that is turned on by a customer when fuel is dispensed into a vehicle. When the customer has completed dispensing fuel, the dispensing switch is turned off which also turns off the pump. A check valve in the product dispensing line prevents fuel from draining out of the product dispensing line when the pump is turned off and maintains the product under pressure in the product dispensing line. A pressure relief valve built into the pump reduces product line pressure to about 11-15 pounds per square inch PSI (159.5-217.5 Bar) after the dispensing switch is turned off.
The product dispensing line also has a line pressure probe installed in a "T" connection in the product dispensing line for sensing product line pressure. In the past, line pressure probes have been designed to detect large scale leaks on the order of three (3) or more gallons (11.36 or more liters) per hour. When detecting large scale leaks, thermally induced pressure variations are relatively insignificant.
Recently, the Environmental Protection Agency (E.P.A.) has required that small scale leaks measured in fractions of a gallon (liter) must be detectable in product dispensing lines. The Environmental Protection Agency (E.P.A.) requires either an annual line tightness test with a required detection rate of "0.1 gallon per hour [0.379 liters] leak rate at one and one-half times the operating pressure," 40 C.F.R. .sctn. 280.44 (b) (1988), or a monthly line tightness test with a required detection rate of "0.2 gallon per hour [0.757 liters] leak rate or a release of 150 gallons [567.75 liters] within a month with a probability of detection of 0.95 and a probability of false alarm of 0.05." 40 C.F.R. .sctn. 280.43 (h) (l) (1988).
With the E.P.A. mandated line tightness test standards, thermally induced pressure variations became significant, and there was a need to distinguish between thermally induced pressure changes and an actual product dispensing line leak.
Some previous hydrocarbon leak detection systems, such as that described in U.S. Pat. No. 4,835,717, have employed a resistive thermal device, Such as a thermistor, to sense product temperature in an effort to compensate for thermally induced pressure changes. Use of a resistive thermal device can create inaccuracies because product temperature is only measured at one location, and product temperature can vary many degrees over the length of a product line.
Some previous hydrocarbon leak detection systems, such as that described in U.S. Pat. No. 4,876,530, have employed a spring loaded piston to operate in a fluid reservoir to supply make up fluid in the event of volume loss by thermal contraction when the pump is off. The pump is operated when the spring loaded piston can no longer supply make up fluid to keep a product line pressurized. Using a piston and reservoir to maintain product line pressure within a predetermined range, generally requires underground product line modifications and the addition of mechanical components that can fail.
What is needed is a programmable pump controller with the capability to determine if product is being dispensed and timed to briefly jog the pump at predetermined intervals to repressurize a product line to compensate for the pressure drop due to thermal contraction to decrease false alarms during a line tightness test.